Poly(arylene thioether) (hereinafter abbreviated as "PATE") represented by poly(phenylene sulfide) (hereinafter abbreviated as "PPS") is used, as a polymer excellent in heat resistance, chemical resistance, etc., in parts for electronic equipment and automobile parts and in a wide variety of other fields. By the way, a carboxyl-containing PATE is not only useful in itself as materials for various kinds of molded or formed products but also expectable to improve adhesion properties. Moreover, it is expected to have wide applications, for example, as compatibilizers upon blending PATE with other engineering plastics, or in production of ionomer resins or preparation of various kinds of derivatives making use of its carboxyl groups
There has heretofore been proposed a production process of a copolymer, which comprises polymerizing dihalo-aromatic compounds including a dihalo-aromatic carboxylic acid with an alkali metal sulfide in the presence of a polar solvent substantially free of any water (Japanese Patent Application Laid-Open No. 305131/1988). However, the present inventors made an investigation with respect to this process and found that the copolymer can be obtained only in the form of powder, and difficulties are encountered on the provision of a copolymer having a sufficiently high molecular weight. In particular, the fine powdered copolymer involves problems of difficulties in its separation and purification from a reaction system upon the production, and of the worsening of labor hygiene and environmental contamination due to flying of the fine powder upon its forming or molding and processing, or of its poor handling properties. Such a copolymer is hence impractical.
On the other hand, it has been proposed to produce a high-molecular weight PATE in the form of granules by subjecting an alkali metal sulfide and at least one dihalo-aromatic compound to two-step polymerization in the presence of water in an amount specifically controlled in an organic amide solvent (U.S. Pat. No. 4,645,826). However, the present inventors made an investigation with respect to this two-step process and found that when polymerization is carried out by using, as said at least one dihalo-aromatic compound, a mixture of a dihaloaromatic compound free of any carboxyl group and a dihaloaromatic carboxylic acid, a copolymer in the form of granules can be obtained in the case where the proportion of the dihalo-aromatic carboxylic acid is extremely small, but only a copolymer in the form of fine powder can be obtained as its proportion becomes larger.